1. Field of the Invention
The present invention relates to rope suspension system for an elevator.
2. Description Of Related Art
Elevators generally include a bed, a motor attached to the bed, a motor shaft, a traction sheave so mounted on the shaft that its plane of rotation is inclined relative to the vertical plane, and at least one diverter pulley. Generally, the suspension ropes in such suspension systems for elevators run from the elevator car to the traction sheave via a route between the traction sheave and the diverter pulley, so that the ropes after passing around the traction sheave run to the diverter pulley via a route proximal to that of the ropes extending towards the traction sheave, the counterweight being suspended on the ropes coming from the diverter pulley.
In current practice, gearless elevators operated at high speeds, such as 2.5 to 10 m/s, use traction sheaves and diverter pulleys provided with rope grooves that have a semicircular cross-section. Such practice necessitates the use of a so-called "double-wrap" suspension, hereinafter referred to as DW suspension, in order to achieve sufficient friction between the ropes and the traction sheave. In DW suspensions, each rope is passed twice around the traction sheave, so that the total angle of contact between each rope and the traction sheave is about 310.degree. to 330.degree.. In fast DW elevators, the suspension ratio is 1:2, by which is meant the rope speed equals twice the car speed. In such elevator systems the ropes going downwards from the traction sheave and diverter pulley are not attached, respectively, to the elevator car and the counterweight but rather are attached to an external fixed structure near the top of the hoistway, the elevator car and the counterweight being suspended on the ropes by pulleys. The high rope speed results in increased noise and vibration in the car. To reduce the noise level, insulation arrangements and their attendant costs are required.
There are many other disadvantages associated with DW suspensions. In 1:2 DW suspensions, the rope has to undergo as many as twelve diversions, which together with the high rope speed causes wear of the ropes and fatigue fractures in the rope wires. In addition, the traction sheave is subjected to a heavy radial load resulting from the large number of rope loops around it, which naturally imposes certain restrictions regarding the choice of a motor. An associated result is the so-called DW effect, in which in certain conditions of wear of the rope grooves, a large force acting between the traction sheave and the diverting pulley and tending to bend the shaft of the traction sheave is developed within the suspension mechanism.
There are also rope suspension systems designed for use with light-weight geared elevators. An example is Finnish patent 56813, which discloses an elevator with a suspension system using at least one diverter pulley to guide the suspension ropes in such manner that the ropes going to the traction sheave cross the ropes leaving it, the angle of contact between the ropes and the traction sheave being within the range of 210.degree. to 250.degree. and the distance between the point of crossing of the ropes and the point of their contact with the traction sheave equalling 1.9 to 0.7 times the traction sheave diameter. The traction sheave is slightly inclined to enable the ropes to run clear of each other at the crossing point. However, the angle between the ropes and the traction sheave is a disadvantage, causing a sideways pull and therefore heavy wear of the ropes.
A similar rope suspension system is proposed in British patent publication 2,148,229, according to which the rope grooves are provided with polyurethane inserts. However, that solution has the disadvantage that the polyurethane wears out quickly due to the lateral pull and the heat generated.
An object of the present invention is to achieve a rope suspension system which reduces of the above-mentioned drawbacks while still preserving substantially the same friction between the ropes and the traction sheave, providing a longer rope life.